A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
To support tighter lithography requirements, accurate correction of the performance of the lithographic apparatus is required. In order to apply more accurate correction functionality, more data/dense sampling of products on a substrate is required to determine a correction set. Using the trade-off between cost of metrology versus accuracy of a correction set, it is common practice that a subset of all products is measured, with the intention to acquire approaching the same level of information as is captured with fully measured products. This is called reduced sampling. Many mathematical approaches exist that support reduced sampling schemes, and these are typically based on geometrical constraints (measurement sites per wafer location).
The effectiveness of a reduced sampling plan to achieve the best accuracy at the lowest possible metrology time/cost is currently determined by a known applied mathematical approach. The applied mathematical approach determines the limitation of the effectiveness of a reduced sampling plan. This is discussed below with reference to FIG. 8.
Current metrology sampling plans are static within a lot of exposed wafer substrates and all measured wafers are sampled with identical sampling plans. Rarely, sampling plans are changed in between lots to adjust for changed state of the exposure and processing equipment. Usually, only a few wafers are measured within each lot to save metrology time and cost.
For CPE (Corrections per Exposure), sometimes wafers are measured with a very dense sampling plan, usually very infrequently (for example once every few weeks).
Problems are:
1) Sampling only a few wafers of each lot may not yield results that are representative for the lot. Wafers outside the regular population that are measured will cause a disturbance in the APC (Advanced Process Control) feedback loop.
2) Wafers outside the regular population may escape detection if not measured.
3) CPE cannot be done very frequently because of the huge metrology cost.